This invention relates to a method of operating an electrooptic modulator.
FIG. 1 illustrates an optical switch 2 that may be used in an optical time domain reflectometer (OTDR) to control propagation of light from a laser diode 4 to a fiber under test 6 and from the fiber under test to a detector 8, such as an avalanche photodiode. The switch shown in FIG. 1 comprises a substrate 10 of Z-cut LiNbO.sub.3 having diffused titanium waveguides 14-18 formed therein. Waveguides 14-18 form two directional couplers 26A, 26B. Electrodes 22, 24 are deposited over an SiO.sub.2 buffer layer (not shown) and an electrode driver 30 is connected to electrodes 22, 24 and controls the state of the directional couplers and hence the state of the switch. The switch has a first state, in which the directional couplers are each in the bar state and light entering waveguide 14, for example, at one end is confined in that waveguide and leaves the waveguide at its opposite end, and a second state, in which the directional couplers are each in the cross state and light entering waveguide 14 from fiber 6 is transferred to waveguide 16 and is then transferred to waveguide 18 so that it is applied to detector 8.
The manner of operation of an electrooptic directional coupler is well understood. In a conventional electrooptic directional coupler, the waveguides are dimensioned and positioned so that the transmission from one waveguide to the other depends on the potential difference between the electrodes in the manner illustrated by the curve A in FIG. 2. At a potential difference of about zero volts, the transmission is a maximum, and it falls off symmetrically as a function of voltage. The coupler is placed in its cross state by holding the electrodes at the same potential, and is placed in the bar state by establishing a potential difference V, which is generally on the order of 25 volts, between the electrodes. Therefore, to place switch 2 in its first state, electrode driver 30 applies a potential +V to electrodes 24, electrodes 22 being grounded, and to place switch 2 in its second state, electrode driver 30 grounds electrodes 24.
Operation of laser diode 4 is controlled by timing control circuit 32. Thus, timing control circuit 32 generates pulses at an interval T of, say, 100 .mu.s, and applies these pulses to a laser driver 36. In response to each pulse, laser driver 36 energizes laser diode 4, which emits a pulse of light having a duration that is typically 100 ns. The waveform of the signal applied to laser driver 36 is shown as curve A in FIG. 3. Timing control circuit 32 also controls operation of electrode driver 30, and hence switch 2. The waveform of the signal provided to electrode driver 30 by timing control circuit 32 might be as shown by the curve B in FIG. 3. For each pulse applied to laser driver 36, timing control circuit 32 applies a pulse 38 to electrode driver 30, which places switch 2 in its first state so that the light pulse from laser diode 4 is coupled through waveguide 14 and coupler 26B into fiber 6.
Following launch of a pulse into fiber 6, back-scattered and reflected light is received from fiber 6. The interval T between pulses applied to laser driver 36 is selected to be longer than the interval during which backscattered and reflected light is received from fiber 6. During intervals in which backscattered light is received from fiber 6, the switch is placed in its second state and light from the fiber under test is coupled to detector 8 through waveguide 14, coupler 26B, waveguide 16, coupler 26A and waveguide 18. The reflected light is generally of less interest than the backscattered light. Also, the reflected light is of much greater intensity than the backscattered light and may overdrive the detector. Therefore, timing control circuit 32 generates pulses 40 in response to reflection signals received from the fiber under test, and these pulses 40 are used to place the switch in its first state in order to isolate the detector from the fiber under test and thereby avoid overdriving of the detector. Therefore, the voltage applied to electrodes 24 is periodic, with a period T, and has a non-zero DC component. The manner in which the timing control circuit functions to mask receipt of reflection signals is well understood by those skilled in the art.
It has been found that some electrooptic directional couplers exhibit a slow drift in their transmission-vs.-voltage curve when the potential difference between the electrodes has a DC component. The curve shifts towards the bar state voltage of the original (unshifted) curve so that the cross and bar states occur at different drive voltages from before. A typical shifted curve is shown at B in FIG. 2. It is apparent from curve B that the drift results in a reduction in the transmission when electrodes 22, 24 are at the same potential and an increase in the transmission when electrode 24 is at a potential +V relative to electrode 22. Accordingly, the drift causes the performance of an optical switch employing electrooptic directional couplers to be degraded.
T. Fujiwara, S. Sato, H. Mori and Y. Fujii, "Suppression of Crosstalk Drift in Ti:LiNbO.sub.3 Waveguide Switches", IEEE J. Lightwave Tech., Vol. 6, No. 6 (1988) discusses the problem of drift and suggests that it is caused by leakage of current through the buffer layer. The leakage takes place under the influence of the voltage applied to the electrodes. In accordance with the disclosure in Fujiwara et al, the relaxation time of the drift is on the order of 20-30 s.
C. M. Gee, G. D. Thurmond, H. Blauvelt and H. W. Yen, "Minimizing dc drift in LiNbO.sub.3 waveguide devices", Appl. Phys. Lett., Vol. 47, 211 (1985), describes use of indium tin oxide (ITO) as the buffer layer of an electrooptic modulator, and reports a dramatic reduction in both short term and long term DC drift of the modulator. However, use of the ITO buffer layer caused substantial attenuation compared to devices with an SiO.sub.2 buffer layer. Also, an increase in the voltage required to achieve full modulation was observed, and it is to be expected that use of an ITO buffer layer would be attended by the disadvantage of increased thermal drift. (See co-pending U.S. patent application Ser. No. 07/460,759 filed Jan. 4, 1990 and still pending.
G. Ramer, C. Mohr and J. Pikulski, "Polarization-Independent Optical Switch with Multiple Sections of .DELTA..beta. Reversal and a Gaussian Taper Function", IEEE Trans. Microwave Theory and Tech., Vol. MTT-30, 1760 (1982), describes a technique for testing an optical switch in which one electrode is grounded and periodic voltage signals are applied to the ungrounded electrode.
U.S. Pat. No. 4,997,245 the disclosure of which is hereby incorporated by reference herein, describes an optical switch comprising two directional couplers and having desirable characteristics of polarization independence and wavelength broadening. However, in order to achieve these advantages, the potential difference between the electrodes of each coupler is at a non-zero bias value when the coupler is in the cross state. Consequently, the DC level of the potential difference between the electrodes is substantially higher than in the case of the switch shown in FIG. 1.